Heterogeneity of a silver surface used in the oxidation of carbon monoxide

Abstract

The mechanism of the oxidation of CO by nitrous oxide and oxygen over a silver catalyst has been studied by an analysis of the transient behaviour at 20 °C. Half of the total surface sites are active for the adsorption of oxygen and one-third of the active sites are active for the decomposition of N₂O, indicating the heterogeneity of the silver surface. At a steady state of CO oxidation in the N₂O + CO and the O₂+ CO systems, the surface for the N₂O + CO reaction is more reduced than that for the O₂+ CO reaction. The activation energy is 14 kcal mol^–1 for the O₂+ CO reaction and 10 kcal mol^–1 for the N₂O + CO reaction, suggesting different rate-controlling steps.

Adsorbed oxygen (presumed to be diatomic) formed from gaseous oxygen and not from N₂O is responsible for the oxidation of CO. An Eley–Rideal-type mechanism is proposed and the rate of oxidation of CO by diatomic oxygen is estimated to be ca. 14 times faster than the rate of decomposition of N₂O. Graphical analysis of the transient response curves obtained for the two reactions leads to the conclusion that the rate of formation of the diatomic oxygen species is the rate-determining step in the N₂O + CO reaction.

Computer simulation of the transient response curves proved the validity of the proposed reaction mechanism and gave estimates of the values of the kinetic parameters (k_j) for all the elementary steps. Comparing the k_j obtained for the two reactions, the dissociation of diatomic oxygen in the O₂+ CO reaction was determined to be one hundred times faster than in the N₂O + CO reaction, again indicating the heterogeneity of the working silver surface.